In conventional arrangements of tunnelling shields having hydraulic thrust rams, a power pack containing a hydraulic pump and motor together with a fluid reservoir are towed behind the shield. The power pack represents a considerable obstruction especially in restricted areas for example for making small diameter tunnels. Moreover, a power pack towed behind the shield is exposed to abuse in use and in particular there are hydraulic pipes extending rearwardly of the shield which are exposed and liable to breakdown under working conditions. The object of the invention is to provide an improved tunnelling shield of the aforesaid type, more suitable for use in excavating small diameter tunnels.